Optical discs such as compact discs (CDs), video compact discs (VCDs) and digital versatile discs (DVDs) are generally used to record data onto a data side thereof with a burner. For convenience, after the burning operation of a disc is finished, the title or other information should be marked on the label side of the disc. An approach for marking the disc utilizes a marking pen to write the information on the disc. If a wrong marking pen is selected, the disc is likely damage because a very thin protective coating on the label side of the disc is vulnerable to chemical or physical attack. In addition, the ink in some kinds of pens may damage the top coating of the disc. In accordance with another approach, an adhesive label is attached onto the label side of a disc. Although the attachment of the adhesive label makes the disc look more professional, there are still some drawbacks. For example, any air bubbles in the adhesive label may cause trouble. If the adhesive label is not perfectly aligned, vibration is likely rendered when the optical disc is rotating in the disc reading apparatus. For reading an unbalanced disc, the rotating speed has to be lowered in order to avoid errors.
A new method for printing the label pattern onto the label side, which is also referred as a light-scribe technology, requires special discs with printable surfaces. The label sides of the discs are coated with a light-sensitive dye that becomes darkened when exposed to the laser light in a specially designed disc burner. After a blank disc is burnt in the usual way, the disc is flipped over and loaded to the burner again. By creating the desired label design on the computer system using a graphics program, the laser light burns the label pattern onto the label side. In comparison with the conventional disc marking method, the light-scribe technology is able to create high-quality label pattern on the disc, for example the effect of serigraphy or grey level.
Typically, the reflectivity of the label side is approximately 10%, which is much lower than the reflectivity of the data side (e.g. approximately 45%). This low reflectivity of the label side has a great influence on the focusing operation of the laser light.
Referring to FIG. 1, a conventional focusing control system of an optical disc drive 100 is shown. An optical disc 110 is driven to rotate by a spindle motor 120. For reading data from the rotating disc, the optical head 10 is driven to move in the tracking direction by a sled motor 130 to perform a seeking operation. Further, the lens 1 of the optical head 10 is driven to move in the tracking direction by a tracking coil 140 to perform a tracking operation. The term “tracking operation” used herein means that the position of the optical head with respect to a selected track is maintained in the proper center position above the selected track. The term “seeking operation” means that the optical head jumps from one track to another track. In addition, the optical head 10 is driven to move in the focusing direction by a focusing coil 145 to perform a focusing operation.
When an electronic signal is generated responsive to an optical signal reflected from the optical disc 110 and received by the optical head 10, the electronic signal is transmitted to a radio frequency (RF) amplifier 150 to be processed into a radio frequency signal RF, a tracking error signal TE and a focusing error signal FE. These signals RF, TE and FE are further processed by a digital signal processor (DSP) 170 to generate three control signals. In response to these three control signals, a first motor driver 160 makes adjustments to output driving forces for driving the sled motor 130, the tracking coil 140 and the focusing coil 145, thereby properly locating the optical head 10 onto the desired track and desired focusing position. Under the control of the digital signal processor 170, a second motor driver 165 outputs a driving force for driving the spindle motor 120, thereby permitting rotation of the disc 110 at a revolving speed.
Please refer to the timing waveforms of FIG. 2(a), in which the voltage variations Fdv outputted by the first motor driver 160 and the focusing error signal FE change as the lens 1 is moved from bottom to top. Typically, the timing waveform FE is also referred as a focusing S curve. This focusing S curve is advantageous for evaluating the timing of enabling the closed-loop control operation. After the focusing error signal FE is filtered by a high pass filter, the shifted focusing error signal FE is shown. For example, when the amplitude of the shifted focusing error signal FE is greater than a specific value Fon and then drops down to a first reference value Fr, the digital signal processor 170 starts a closed-loop control operation so as to result in a sub-beam addition signal SBAD. When the sub-beam addition signal SBAD has up-crossed a threshold T for a predetermined period of time, it is discriminated that the focusing operation is completed.
Referring to FIG. 2(b), the optical head 10 has three optical sensors. The central optical sensor has four light receiving parts A, B, C and D for respectively receiving the main beam reflected from the disc 110. Whereas, the bilateral optical sensors have four light receiving parts E, F, G and H for respectively receiving the sub-beam reflected from the disc 110. The sub-beam addition signal SBAD is the summation of the overall light intensity received by the receiving parts E, F, G and H, i.e. (E+F+G+H), where E, F, G and H are light intensities received by the regions E, F, G and H, respectively. The focusing error signal FE is substantially a difference between the summation of the overall light intensity received by the receiving parts A and C and the summation of the overall light intensity received by the receiving parts B and D, i.e. (A+C)−(B+D), where A, B, C and D are light intensities received by the regions A, B, C and D, respectively. When the light emitted by the light source is perfectly focused on the desired point, the overall light intensity received by the receiving parts B and D will be equal to that the overall light intensity received by the receiving parts A and C, i.e. FE=(A+C)−(B+D)=0. In another case that the value of (A+C)−(B+D) is minus, a focusing position is above the perfect position. Whereas, the positive value of (A+C)−(B+D) indicates a focusing position below the perfect position.
As previously described, the reflectivity of the label side of the disc 110 is approximately 10%. Due to this low reflectivity, the light intensities received by the receiving parts A, B, C and D are very weak. Since the focusing error signal FE is determined according to the difference between (A+C) and (B+D), the amplitude of the focusing error signal FE or the shifted focusing error signal FE is very low. For enlarging the amplitude of the focusing error signal FE, the gain of the RF amplifier 150 may be increased. However, the noise contained in the focusing error signal FE is also increased as the gain of the RF amplifier 150 is increased. Under this circumstance, since the focusing S curve becomes very smooth, the timing of enabling the closed-loop control operation is evaluated with difficulty or mistake. Such difficulty or mistake is therefore insufficient to provide a good focusing control efficacy and becomes problematic in marking the label side of the disc.